What might be the consequence(s) if the collector/case of TO3 output transistor was shorted to ground?

[62vauxhall]

If I may pick a brain or three,

This might be a basic "Electronics 101" question and the component make/modle may be irrelevant.

I've spent some time dabbling inside a free NAD 7225PE receiver. The power amp section has a problem. Both channels work and the volume it produces is equal on both sides. But the audio is quite distorted, again equally on both sides.

After following the service manual proceedure for adjusting bias - removing solder bridge and using designated test points, I saw that rotating the adjustment pots have no effect. The reading continued to show 0.0mv.

I've pulled some of the non-TO92 transistors to test out of circuit and so far all seem to be OK including the TO3 outputs. Which in this case are one pair per side of an MJ2955 and a 2N3055.

There has been previous work done. One channel's pair of output transistors have been replaced.

As I cleaned off the old thermal paste, I noticed that two of the insulating sleeves used to attach the TO3's have pieces broken off. Also that 2 of 4 mica insulators had cracks.

If the collector/case was shorted to ground, how would that manifest? Destruction of the transistor or distortion of audio?

The schematic is one channel of the receiver and the TO3's are Q415 and Q417.

 

[20tajk7]

Shorting the output transistors cases to ground will short the +/-41.1V power supply, burn the R455 resistor and blow the main fuses.

 

[WOPL Sniffer]

Which could include shorting/opening/weakening the junctions of the output transistors and many other components. It's a shock (no pun intended) to the surrounding system.

 

[stetter]

Which could include shorting/opening/weakening the junctions of the output transistors and many other components. It's a shock (no pun intended) to the surrounding system.

With the cracked mica and paste I have found transistor bad and more like Both of the answers above

 

[62vauxhall]

Glen called, shortly after I posted and straightened me out.

 

[WOPL Sniffer]

So anything we said can't happen????

 

[62vauxhall]

Oh yes, it could.

 

[markthefixer]

blooie........

you'll KNOW it.....

 

[WOPL Sniffer]

 

[WOPL Sniffer]

 

[WOPL Sniffer]

Didn't some PL amps burn down a shack or two when the outputs shorted???

 

[laatsch55]

Yep..

 

[nakdoc]

Typical amp failure in the small NADs is outputs and drivers, and the bias transistor, plus a resistor or two. Q409 is shorted. My guess is one of the drivers is open causing the distortion.

 

[FredR]

https://www.dictionary.com/browse/annihilation

 

[62vauxhall]

Typical amp failure in the small NADs is outputs and drivers, and the bias transistor, plus a resistor or two. Q409 is shorted. My guess is one of the drivers is open causing the distortion.

Thanks! Q409, and it's other channel counterpart, I have not checked yet.

 

[Gepetto]

Total Protonic Reversal...

 

[J!m]

Important safety tip.

 

[62vauxhall]

Q409 (BD139) was bad so I replaced it and one burnt resistor. Also stuck a new BD139 at Q410 in the opposite channel.

One of the outputs is not cooperating when it's screws are tightened - the DBT does not get that dim. So I have no faith in the TO3's. I am going to replace all o them - two each 2N3055 & MJ2955. Digikey don't got so coming from Mouser.

They have both parts from Onsemi on hand at $7 and $8 respectively. Their website lists MJ2955 from Central Semiconductor as well. A PB FREE version that costs about half what the Onsimi sells for.

Out of curiosity, why would that be? I had the notion a lead free offering would be more than a conventionial offering. Sort of like "organically grown" food is two or three times the cost of regularly grown food even though producers don't have the expense of fertilizers, pesticides or hormones.

I'm going to ditch the mica insulators and get some sil pads. It was a slog reading through the list of those things but Aavid branded ones for $1.05 look like they should do. Considering what they are, that seems like a lot but others are pricier.

 

[Gepetto]

Hi Gary
The G after the 2N3055 and MJ2955 from ON Semi means RoHS/Lead free

the price difference is ON Semi is a premier brand and Central is not

ON Semi is the old Motorola. Motorola/ON Semi makes the best power semiconductors on earth and they know that

 

[62vauxhall]

I see. Thanks for the explanation!